The present invention is directed to an optical modulator and, more particularly, to an optical modulator that includes a Mach-Zehnder interferometer having a resonator coupled to one arm that increases the optical length of that arm and that also reduces the amplitude of a drive voltage signal required to introduce a phase-shift into an optical signal propagating through the arm to which the resonator is coupled.
A typical Mach-Zehnder modulator includes an interferometer having an input waveguide, two arms that branch from the input waveguide, and an output waveguide at the junction of the two arms. An optical signal is directed into and propagates in the input waveguide, and is split between the two arms so that approximately one-half of the input optical signal propagates in each of the interferometer arms. A drive voltage is applied to one arm of the interferometer which changes the effective refractive index of that arm and introduces a phase-shift in an optical signal propagating in that arm. The phase-shifted optical signal combines with the non-phase-shifted optical signal at the output waveguide and produces amplitude modulation in the optical signal due to phase mismatch between the signals and the fact that parts of the two optical signals interfere both constructively and destructively. The output of the modulator is thus an amplitude modulated optical signal. A relative phase-shift between the optical signals in the two arms of approximately xcfx80 is required to achieve large signal modulation (i.e., the ability to switch the output of the modulator between on and off states). The voltage required to introduce a phase-shift of approximately xcfx80, Vxcfx80, is typically between 5 and 10 volts AC (VAC).
Prior art Mach-Zehnder modulators, such as those made from Lithium Niobate, are relatively large (e.g., about 10-60 millimeters long, measured generally as the length of the arm) and require a relatively high Vxcfx80 (e.g., between 5 and 10 VAC) because the electro-optic effect in such modulators is weak. Semiconductor Mach-Zehnder modulators can be smaller (e.g., about 1-20 millimeters long) than those constructed of Lithium Niobate due to stronger electro-optic effects for some semiconductor materials, when compared with Lithium Niobate. However, approximately 3 mm length of waveguide is still required to introduce a phase-shift of xcfx80 to an optical signal, and a drive voltage of between approximately 0.5 and 2 VAC may still be required.
There thus exists a need in the art for a modulator that overcomes the above-described shortcomings of the prior art.
The present invention is directed to a low drive voltage optical modulator that includes a Mach-Zehnder interferometer having a resonator located near one of its arms.
A Mach-Zehnder interferometer having an input waveguide that splits to form first and second arms, which converge to form an output waveguide. A resonator having a diameter of less than or equal to approximately 50 xcexcm is located near one of the first and second arms and operatively coupled thereto across a gap having a width of less than or equal to approximately 0.5 xcexcm. When an optical signal is directed into the input waveguide, that optical signal is split approximately between the arms; with a first portion of the optical signal propagating in the first arm and a second portion of the optical signal propagating in the second arm. The resonator is tuned to a predetermined wavelength (preferably matched to the wavelength of the optical signal directed into the waveguide by an optical source) and a portion of the optical signal propagating in the arm near the resonator is coupled to the resonator. An AC voltage applied to the resonator may cause the refractive index of the resonator to change, which may cause the optical length of the resonator to change thus imparting a phase-shift in the optical signal propagating therein. Thus, the optical signal propagating in the arm near the resonator, when viewed at a location optically downstream from the resonator, is phase-shifted with respect to the optical signal propagating in the other arm. When the phase-shifted signal recombines with the non-phase-shifted signal at the junction of the two arms (i.e., at the output waveguide), the optical signal propagating in the output waveguide and emerging therefrom is amplitude modulated because the optical signals emerging from the respective arms will interfere constructively and destructively due to the phase mismatch between those signals.
In another embodiment of the present invention, a respective resonator is located near both arms of the Mach-Zehnder interferometer. An AC drive voltage of approximately equal amplitude, but opposite polarity, is applied to the resonators to introduce opposite phase-shifts in the optical signal propagating through the two arms, thereby doubling the amount of phase-shift possible with a given voltage.
In yet another embodiment of the present invention, a low drive voltage optical modulator comprises a Mach-Zehnder interferometer having an input waveguide, first and second arms connected to the input waveguide, and an output waveguide connected to the first and second arms. The modulator of this embodiment also includes a phase-shifter that is operatively coupled to the first arm across a gap and that causes a predetermined phase shift in an optical signal propagating in the first arm.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure herein, and the scope of the invention will be indicated in the claims.